Proteins are large, complex molecules that have many functions within organisms, from catalysing reactions to being key components in cellular structure.
Proteins are long chains of amino acids, which are held together by peptide bonds. These chains are called polypeptides. A protein can consist of just one polypeptide or multiple.
Amino Acids
The general structure of an amino acid is shown below:
All amino acids consist of a central carbon atom bonded to a hydrogen atom, an amine group (NH2), a carboxyl group (COOH) and a variable R group.
The R group varies depending on the specific amino acid and this group determines the function of the amino acid. In total there are 20 different amino acids that are used to make the proteins found in humans.
The Formation of a Peptide Bond
To form a polypeptide amino acids are joined together by a peptide bond. Peptide bonds are formed in condensation reactions between the carboxyl group of one amino acid and the amine group of another.
In this reaction a molecule of water is released. In contrast, peptide bonds are broken by hydrolysis reactions (the addition of a water molecule).
If only two amino acids join together we get a dipeptide. The diagram below shows the formation of a dipeptide:
A polypeptide is when more than two amino acids join together.
The Structure of Proteins
The structure of a protein can be broken down into four different levels:
Primary Structure
This is the most basic structure of a protein. It is simply the sequence of amino acids in the polypeptide chain.
Secondary Structure
Hydrogen bonds form between the amino acids in the polypeptide chain. These bonds cause the primary structure to fold or coil.
If the protein folds it forms what is known as a beta pleated sheet. If the protein coils it forms an alpha helix.
Tertiary Structure
The secondary structure continues to fold or coil and more bonds form in various places. This creates a specific 3D shape.
If the protein only consists of one polypeptide chain, this 3D structure will be the final structure of the protein.
Quaternary Structure
Many proteins consist of multiple polypeptide chains. The different chains bond and pack together in a specific way to form a complex structure.
The way in which the chains are held together determines the final quaternary structure of the protein and its properties.
Please leave any questions you have in the comments below and let me know if you would like a more detailed post on the structure of proteins. For more biology posts click here and follow me on Instagram and Pinterest to stay up to date with the latest posts. For more posts click on one of the categories below or use the search bar at the bottom of the page.